visible light - Did Newton argue that particles speed up when entering a more dense medium?

Statement:

Newton argued that particles speed up as they travel from air into a dense, transparent object, such as glass.

From this source, I gather that he did argue that the light particles sped up when entering a more dense medium. However, it just doesn't make sense. I thought they seemed to slowed down.

Answer

Like all good physicists Newton proposed a hypothesis for why light refracts when it crosses a boundary between different refractive indices. And his hypothesis makes a certain amount of sense:

If the initial velocity of the light is $v$, then the velocity parallel to the boundary is:

$$ v_p = v\sin i $$

and the velocity normal to the boundary is:

$$ v_n = v\cos i $$

Newton's hypothesis for the bending of the light ray is that the component of velocity normal to the boundary increases in the denser medium (while the parallel component is unchanged). Since the parallel component is unchanged we have:

$$ v'_p = v_p = v\sin i = v' \sin r $$

And therefore:

$$ \frac{v'}{v} = \frac{\sin i}{\sin r} $$

which is exactly the reciprocal of what Snell's law predicts!

Until some way was found to measure $v'$ the theory wasn't testable, but it was predictive and indeed gave the correct values for $r$ given any initial value $i$ and constant $v'/v$. Not surprising as it is Snell's law - just derived from a mistaken assumption.

Until some way was found to measure $v'$ Newton's theory was as good as any other. The problem is that when the measurements were done $v'$ turned out to be less than $v$. Explaining this required treating light as a wave rather than a particle. However the measurement wasn't done until nearly two centuries after Newton's suggestion.

A critic could point out that the change in the normal component of velocity, i.e. the acceleration of the light towards the boundary, would have to depend on angle for the theory to work, and no mechanism for this was suggested. But then Newton suggested no mechanism for his theory of gravity and it took nearly three centuries for Einstein to provide one and show that in this case Newton was right.